1. Field of the Invention
The present invention relates to a shock absorber incorporated in a suspension system of a vehicle such as an automobile, and more particularly, to a damping force control system for such a shock absorber.
2. Description of the Prior Art
In Japanese Patent Laying-open Publication No. 62-168704 and Japanese Utility Model Laying-open Publication No. 62-11010 there is disclosed a damping force control system for a shock absorber incorporated in a suspension system of a vehicle such as an automoble which is adapted to switch over the damping force of the shock absorber to a high level when the relative displacement between the wheel and the vehicle body has increased beyond a predetermined limit value, maintaining the shock absorber at the high damping force operation until the relative displacement decreases substantially to zero. In such a damping force control system, when the vehicle runs on a rough road such as an unpaved road, the switching over of the damping force between a low level and a high level occurs at a very high frequency, causing a problem that the durability of the shock absorber and the actuator therefor is low.
In view of such a problem it is contemplated to maintain the shock absorber at the high damping force operation for a predetermined period when the relative displacement between the wheel and the vehicle body has increased beyond a determinate value. However, if the period for maintaining the shock absorber at the high damping force operation is predetermined, it will occur that the shock absorber is operated for a too long period at the high damping force on a road having smaller roughness, thereby unnecessarily lowering the comfortableness in riding, while on a road having larger roughness the shock absorber is operated at the high damping force for a too short period to ensure the stability in maneuvering.